The invention relates to telephone line equipment and more particularly to equipment for use in an electromechanical or electronic telephone exchange. It comprises a two-wire to four-wire conversion circuit, a circuit for supplying d.c. (at 48 V) to the subscriber's line, and a closed loop detection circuit. The equipment may also, in a known manner, comprise a ringing signal transmission circuit (70 V and 50 Hz) and a device for limiting the ringing current in the event of a short-circuit on the subscriber's line.
In conventional subscriber's line equipment, two-wire to four-wire conversion is normally accomplished by a telephone transformer, or sometimes by a (different kind of) transformer which is simpler and less bulky than a telephone transformer, together with a 48 V supply bridge, a current-limiting device, a closed loop detecting means, a switching relay on the ringing current circuit, and a device for limiting the last-mentioned current. The aforementioned equipment has the disadvantages of being bulky and expensive.
The equipment according to the invention does not have any of the aforementioned disadvantages, since it does not comprise a telephone transformer or any other kind of transformer, but uses a chopper, which is compact and less expensive.